In many applications of single mode optical waveguides, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the waveguide to have an azimuthal asymmetry of the refractive index profile.
A slight improvement in the polarization performance of single mode optical waveguides is achieved by distorting the fiber core symmetry as a means of decoupling the differently polarized waves. Two such optical fiber waveguides are disclosed in U.S. Pat. No. 4,184,859 and in the publication by V. Ramaswamy et al., "Influence of Noncircular Core on the Polarisation Performance of Single Mode Fibers," Electronics Letters, Vol. 14, No. 5, pp. 143-144, 1978. However, the Ramaswamy publication reports that measurements on borosilicate fibers with noncircular cores indicate that the noncircular geometry and the associated stress-induced birefringence alone are not sufficient to maintain polarization in single mode fibers.
The invention disclosed in U.K. patent application GB No. 2,012,983 A is based upon the recognition that orthogonally polarized waves are more efficiently decoupled in a waveguide that is fabricated in such a manner as to deliberately enhance stress-induced, or strain birefringence. That patent teaches that such behavior is accomplished by introducing a geometrical and material asymmetry in the preform from which the optical fiber is drawn. The strain-induced birefringence is introduced by at least partially surrounding the single mode waveguide by an outer jacket having a different thermal coefficient of expansion (TCE) than that of the waveguide and a thickness along one direction that is different from its thickness along a direction orthogonal to the one direction. For example, the preform may be a three-layered structure comprising an inner core region surrounded by a cladding layer which is in turn surrounded by an outer jacket layer having a TCE different than that of the cladding layer. Diametrically opposed portions of the outer layer are ground away, and the resultant preform is drawn into a fiber approximating a slab configuration in which the thicknesses of the outer jacket layer are different in two orthogonal directions. A similar result can be accomplished by constructing the preform from an inner core region, a cladding region and two outer jacket layers oppositely disposed along the longitudinal surface of the preform. Difficulty can be encountered in the manufacture of that type of preform since stress is built up in the outer layer. When grinding the outer layer or when cutting slots therein, the built-up stress has a tendency to cause the preform to break. Assuming that a fiber can be drawn from the preform, the stress-forming outer layer is far removed from the fiber core, and therefore, the effect of the stress on the core is minimal.
In one embodiment of GB No. 2,012,983 A represented by FIGS. 10-15, a relatively thick substrate tube forms the outer portion of the optical fiber. In order to impart to the fiber the desired characteristics, either the inner or outer surface of the substrate tube is non-circular. Because at least a portion of the substrate wall must be relatively thick, the efficiency of deposition is adversely affected. Also, since the substrate tube forms the outer, compressive layer of the fiber, commercially available tubes may not be usable in the process unless they fortuitously possess the desired expansion and/or viscosity characteristics of the resultant fiber outer layer.
In a fiber such as that illustrated in FIG. 12 of GB No. 2,012,983 A, the outer layer 60 of cladding is referred to herein as the stress cladding. It has been found that the stress .sigma. at the core of a circularly symmetric single mode optical waveguide fiber is equal to the product of f.times.g where f is a function of geometrical factors and g is a function of glass factors. The function f is given by the equation ##EQU1## where A.sub.sc is the cross-sectional area of the stress cladding and A.sub.f is the total cross-sectional area of the fiber. The function f can therefore have a value such that 0&lt;f&lt;1. The function g is given by the equation ##EQU2## where E is the effective elastic modulus of the fiber, .DELTA..varies. is the difference between the TCE of the stress cladding and the TCE of the remainder of the fiber, .DELTA.T is the difference between the lowest set point of the glasses of which the fiber is comprised and room temperature and .nu. is Poissons ratio. Since the aforementioned definition of stress .sigma. generally applies also to non-symmetrical fibers such as those disclosed in GB No. 2,012,983 A, it is necessary to maximize f to obtain the greatest core stress and thus obtain the greatest stress birefringence. Values of f greater than 0.9 should be achieved to provide maximum values of stress birefringence. The need to maximize function f is recognized in GB No. 2,012,983 A as evidenced by equations (7) and (8) thereof.
Another art-recognized design criteria for single mode optical waveguides is concerned with minimizing loss. A common method of forming single mode optical waveguide preforms is illustrated in FIG. 11 of GB No. 2,012,983 A which shows a plurality of vapor deposited layers on the inner surface of a substrate tube. The purity of the substrate tube is generally not as high as that of the vapor deposited glass. Therefore, the vapor deposited core glass is isolated from the substrate tube by a layer of vapor deposited optical cladding glass of sufficient thickness. For a single mode fiber having a core cross-section which is circular or nearly circular, the radius r.sub.s of the optical cladding should be at least five times the radius r.sub.a of the core. This estimate is based on the findings reported in the publication: Electronics Letters, Vol. 13, No. 15, pp. 443-445 (1977). For fibers having cores of oblong cross-section, this relationship lacks meaningful significance. In such a fiber, the extent of the optical cladding is better described in terms of its thickness. Since the size of a single mode core is related to the transmission wavelength .lambda., the thickness of the optical cladding can also be specified in terms of .lambda.. The aforementioned cladding radius to core radius ratio implies that the thickness of the optical cladding be at least about 20.lambda.. When a single mode waveguide is designed in accordance with this criteria, loss associated with cladding thickness is limited to an acceptably low value.
The following analysis of GB No. 2,012,983 A is made by taking into consideration, inter alia, the specific embodiment described in conjunction with FIGS. 10-12 thereof. The fiber of that embodiment will satisfy the requirement that the ratio A.sub.sc /A.sub.f exceed 0.9 except when the substrate tube is completely filled with internal layers during the process of making the preform from which the fiber is drawn. This aforementioned exception is, of course, an impossibility. Since the substrate tube cannot be completely filled during the internal layer deposition process, the total thickness of the internal layers is limited by the internal diameter of the substrate tube. It is well known that the core diameter of a step profile single mode fiber is about 3 to 10 .mu.m. The outside diameter of the fiber is typically about 125 .mu.m. If the preform described in GB No. 2,012,983 A is formed in accordance with conventional practice so that the ratio A.sub.sc /A.sub.f exceeds 0.9, the thickness of the optical cladding layer will be less than 20 .lambda. at conventional wavelengths. Thus, the excess fiber loss due to insufficient optical cladding thickness will not be sufficiently low for many applications.